Estuarine and Freshwater Habitat – SS2 Biology Lesson Note (Third Term)

Estuarine and freshwater habitats are two aquatic habitats that differ mainly in salt content. An estuary is where fresh water meets and mixes with sea water, so its salinity fluctuates. A freshwater habitat is formed mainly from inland waters and contains very low or no salt. Each has plants and animals with features that suit it.

Behavioural objectives

By the end of the lesson, students should be able to:

• define an estuary and name examples of estuarine habitats;
• state the characteristics of estuarine and freshwater habitats;
• distinguish between lotic and lentic fresh waters;
• describe the adaptive features of common plants and animals of estuarine and freshwater habitats;
• construct simple food chains for estuarine and freshwater habitats.

What are estuarine habitats (brackish water habitats)?

An estuary is a body of water formed at the coast where freshwater flowing towards the sea mixes with the sea (salt water) flowing inland. Estuarine habitats include deltas, lagoons and bays.

Characteristics of estuaries

1. The salinity fluctuates.
2. The specific gravity is less than that of the sea.
3. They have high turbidity due to frequent disturbances. Hence the rate of photosynthesis and respiration by organisms reduces.
4. The water is shallow.
5. They have a low diversity of species compared to marine habitats.
6. They have high levels of nutrients.
7. They have low oxygen content, hence anaerobic activities are common.

Plant distribution and adaptation in estuaries

Plants found in estuaries include plankton, algae, and red and white mangroves. They have the following adaptive features:

1. Plankton (diatoms) have:
   • air spaces in their tissues;
   • rhizoids for attachment to rock shores;
   • air bladders for buoyancy.
2. Algae have:
   • chlorophyll for photosynthesis;
   • small size or large surface area for floating in water.
3. Red mangrove has:
   • stilt roots with rootlets that have air spaces for air conduction to the root tissues, and for support to prevent the plant from being washed away by the tide;
   • seeds which germinate while they are still on the parent plant, thus preventing the seedlings from being carried away by water currents.
4. White mangrove has pneumatophores (breathing roots) for gaseous exchange.

Animal distribution and adaptation in estuaries

Animals found in estuaries include mosquitoes, crustaceans, molluscs, worms and fishes. They survive by having the following features:

1. Mosquito larvae and pupae have breathing trumpets for gaseous exchange.
2. Crustaceans and water snails burrow into the mud to escape predators, strong waves or tides.
3. Worms have a strong, protective and impermeable covering against high salinity.
4. Mudskippers have fins for crawling on land and swimming in water.
5. Fishes have fins for movement and swimming bladders for buoyancy.

Food chains in estuarine habitats

• Detritus → worms → snails → birds
• Diatoms → shrimps → fishes
• Diatoms → small fish → sharks → man

What are freshwater habitats?

A freshwater habitat is a body of water formed mainly from inland waters, and it contains very low or no salt. Fresh water is of two types based on its mobility:

1. Lotic fresh waters: These are running waters flowing continuously in a specific direction, e.g. rivers, springs and streams.
2. Lentic fresh waters: These are stagnant waters which do not flow, e.g. lakes, ponds, puddles, swamps and dams.

Characteristics of freshwater habitats

1. It contains little or no salt. Salinity is 5 parts per thousand, i.e. 0.5%.
2. It is small in size.
3. Oxygen concentration is high, being available in all parts of the water body, especially at the surface.
4. The water is shallow, hence sunlight penetrates to the bottom.
5. The temperature varies with seasons and depth.
6. It has seasonal variation, decreasing or drying up in the dry season and increasing in the rainy season.
7. Water currents affect the distribution of organisms, salts and gases, especially in lotic fresh waters.

Plant distribution and adaptation in fresh water

Plants of fresh water include water lilies, spirogyra, water lettuce and water weeds. They have the following adaptive features:

1. Water lily has:
   • air bladders;
   • expanded tips and light weight, which keep it afloat.
2. Spirogyra has a mucilaginous cover for protection.
3. Water lettuce has hairs on its leaves to trap air and keep it afloat.
4. Water weed (elodea) has a long and flexible petiole for swinging with water currents.

Animal distribution and adaptation in fresh water

Animals of freshwater habitats include protozoa, ducks, pond skaters, hydra and fishes. Their adaptive features include:

1. Protozoa have a contractile vacuole for osmoregulation in water.
2. A duck has webbed feet for locomotion and a serrated beak for sieving food in water into its mouth.
3. Hydra has a slippery surface, hooks and suckers for attachment to water particles.
4. Pond skaters have long legs for skating on water surfaces.
5. Fishes have swim bladders for buoyancy and gills for respiration.

Food chains in freshwater habitats

• Diatoms → fish fry → tilapia
• Spirogyra → tadpoles → carps → kingfish
• Algae → mosquito larva → small fish

Evaluation

1. Define an estuary and name three estuarine habitats.
2. State four characteristics of estuaries.
3. Distinguish between lotic and lentic fresh waters, giving two examples of each.
4. State one adaptive feature each of red mangrove and white mangrove.
5. Name the adaptive features that help a duck to move and to feed in water.

Assignment

1. Buoyancy in salt water is ensured by the following except A. divided leaves B. chlorophyll C. floating devices D. air bladder
2. The mucilaginous cover in seaweed and spirogyra is mainly for A. protection B. osmoregulation C. avoiding desiccation D. feeding
3. Which of the following is not a freshwater habitat? A. puddle B. swamp C. stream D. sea
4. Which of these is not an adaptive feature in a marine habitat? A. bladder for floating B. hold fast for attachment C. fur to prevent water loss D. rhizoid for attachment to rock
5. The following are characteristics of freshwater habitats except A. low salt content B. high salinity C. shallow water D. can be stagnant or running water
6. In a tabular form, state five differences between freshwater and marine habitat.